This invention relates to a recording and/or reproducing device for detecting the servo pattern pre-recorded at a pre-set interval on a disc-shaped recording medium, such as an optical disc, magneto-optical disc or a magnetic disc, and for generating channel clocks based on the detected information on the servo patterns.
There has hitherto been known an optical disc of the sample servo system as shown in JP Patent KOKAI (Laid-Open) Publication No. 3-156774 (1991).
With the sampled servo system optical disc device, the channel clocks, which are clock signals for sampling data used during recording or reproduction, need to be phased correctly.
FIG. 1 shows an arrangement of a clock detection system for the optical disc as applied to a reproducing device for a magneto-optical disc.
The magneto-optical disc employed with the reproducing device shown in FIG. 1 is explained by referring to FIG. 2. The magneto-optical disc includes a number of servo areas each having recorded therein servo patterns each consisting of a pair of wobble pits 200 and a clock pit 201. Each pair of the wobble pits is offset towards the inner and outer peripheries from the center of a concentric track TR and the associated clock pit is formed on the center of the track TR intermediate between the wobble pits.
960 of the servo areas, for example, are formed for each circular turn of the track, with an area defined between a given servo area to the next servo area being a data area. Recording data modulated in a pre-set manner are photomagnetically recorded in the data area along with synchronization data, address data and the like.
Referring to FIG. 1, before proceeding to reproduction of recording data recorded on the magneto-optical disc, a servo pattern detection mode is set, in which the reproducing device for the magneto-optical disc causes the magneto-optical disc to be run in rotation by a spindle motor 100 for phase-matching the servo pattern detection data with the channel clocks.
During the servo pattern detection mode, the optical system 102 causes a laser beam to be radiated on the magneto-optical disc 101 to detect the return light beam. The recording data and the servo patterns recorded on the magneto-optical disc 101 are reproduced by photo-electric conversion of the return light beam for generating playback signals which are supplied to an amplifier 105.
The amplifier 105 amplifies the playback signals with a pre-set gain to supply the amplified playback signals to an A/D converter 106. The A/D converter 106 is fed with channel clocks from a voltage controlled oscillator VCO 111, the oscillation frequency of which is changed in dependence upon the voltage supplied thereto. The A/D converter 106 samples and digitizes the playback signals by the channel clocks for generating playback data which is supplied to a latch circuit 107.
Each servo pattern is made up of a pair of wobble pits 200 and a clock pit 201 disposed intermediate between the wobble pits 200 on the center of the track TR, as shown in FIG. 2. Thus the waveform of the reproduced data (sometimes referred to herein as replay data) for the servo pattern is such a waveform in which the waveform portions for the wobble pits 200 lower in level than the waveform portion for the clock pit 201 are present on both sides of the waveform for the clock pit 201 with the waveform for the clock pit 201 as the center.
If, when the channel clock from the VCO 111 is of a correct phase, the channel clocks start to be counted after detection of the servo pattern, and the playback data from the A/D converter 106 is latched as the count value becomes equal to 277 to 279, 282 to 284 and 287 to 289, the servo pattern may be latched correctly.
To this end, a servo pattern detection circuit, not shown, compares a pre-stored servo pattern and the playback data from the A/D converter 106 with each other for detecting the servo pattern and transmits the detected .pulse to a counter, not shown. The counter is reset by the servo pattern detection pulse and, from this time on, starts counting the channel clocks. The counted value is supplied to a latch pulse outputting circuit, not shown. The latch pulse outputting circuit transmits a latch pulse to the latch circuit 107 when the count values become equal to e.g. 277 to 279, 282 to 284 and 287 to 289, as shown in FIG. 3a.
The latch circuit 107 latches the replay data by each latch pulse to latch the servo pattern replay data as shown in FIG. 3a for generating latch data at points a1, a0, a2, b1, b0, b2, c1, c0 and c2. These latch are supplied to a phase generator 108.
The phase comparator 108, thus fed with the servo pattern replay data, finds the phase errors between the channel clocks and the servo pattern replay data, by taking advantage of left-to-right symmetry of the three waveform portions, based on the level differences from points a0, b0 and c0, the center points of the respective waveform portions, to points a1, a2, b1, b2, c1 and c2, the hump or shoulder points spaced apart by one channel clock ahead and back of the center points a0 to c0, as shown in FIG. 3a, in accordance with the following equation 1 EQU phase error data=1/2[(a2-a1)+(c2-c1)] (1)
to transmit the resulting phase error data to a D/A converter 109.
Meanwhile, the servo pattern replay data are employed for forming tracking error signals, tracking polarity signals (TPOL), which go to high level when an error is within .+-.1/4 track from the track center, one-eighth off-track signals, which go to high level when an error exceeds .+-.1/8 track from the track center, a detection signal of a mean wobble pit level detection signal, and a mirror area level detection signal, in accordance with the following equations: EQU tracking error signal=[(c0-a0)] (2) EQU 1/8 off-track signal=[(b0&lt;a0)+(b0&lt;c0)] (3) EQU TPOL=[b0&gt;1/2(a0+c0)] (4) EQU mean wobble pit level detection signal=1/2(a0+c0) (5) EQU mirror area level detection signal=d0 (6)
respectively, where the mirror area is an area provided between the servo pattern and the data area.
The D/A converter 109 converts the phase error data into analog signals to form phase error signals which are supplied to a phase compensation circuit 110.
The phase compensator circuit 110 is constituted by e.g. a low-pass filter which frees the phase error detection signal of high-range noise components by way of phase compensation of the phase error signals to supply the resulting data to the VCO 111.
The VCO 111, which has its oscillation frequency varied in dependence upon the phase error signals, controls the channel clock phase so that the phase error of the channel clocks with respect, to the servo pattern replay data becomes equal to zero. These channel clocks are supplied to both the A/D converter 106 and the demodulating circuit 103.
As may be seen from the foregoing, the portion of the recording and/or reproducing device for the magneto-optical disc which forms the channel clocks constitutes a so-called phase-locked loop which outputs channel clocks phase-synchronized with the phase of the servo pattern replay data,
When the channel clocks phase-synchronized with the phase of the servo pattern replay data start to be outputted, that is when the phase capturing or phase locking of the servo pattern replay data is terminated, the reproducing device for the magneto-optical disc terminates the servo pattern detection mode to shift to the replay mode of reproducing recording data recorded on the magneto-optical disc 101.
While in the replay mode, the reproducing device for the magneto-optical disc reads the recording data recorded on the magneto-optical disc 101. As mentioned above, the replay signals from the optical system 102 are supplied via the amplifier to the A/D converter 106, while being supplied to the demodulating circuit 103.
The A/D converter 106 generates the replay data based on the channel clocks to supply the data to the latch circuit 107. This allows the servo patterns to be latched to generate the phase error detection data based on which variable control of the output frequency of the VCO 111 is performed repeatedly.
The demodulating circuit 103 demodulates the replay signals from the amplifier 105, based on the channel clocks, and outputs the demodulated replay signals via an output terminal 104.
Since the channel clocks are phase-synchronized with the servo pattern detection data phase, as described above, the replay signals may be correctly sampled and demodulated by the demodulating circuit 103 to assure outputting of correct replay signals.
It is noted that, if the free-running frequency of the VCO 111 is deviated from the desired frequency or larger phase errors are produced after phase capturing and hence the frequency of the channel clocks generated by the VCO 111 is fluctuated significantly, the channel clock phase is deviated significantly from the phase of the servo pattern replay data, such that the latching circuit 107 performs the latching at the positions deviated from both humps or shoulders of the data waveform portions of the wobble pits 200, as shown for example in FIG. 3b. As a result thereof, correct phase error detection data cannot be formed by the phase generator 108, so that correctly phased channel clocks, that is channel clocks synchronized with the phase of the servo pattern detection data, cannot be outputted by the VCO 111.
The reason is that the ability of the phase error detection circuit 105 in forming the phase error detection data is narrow, as shown in FIG. 4, so that, if larger phase errors as referred to above are produced, the phase error detection circuit 105 is unable to generate the phase error difference data corresponding to the larger phase errors.
Consequently, the above-described reproducing device for the magneto-optical disc is provided with high precision clock generating circuit, such as a voltage-controlled quartz oscillator, as the VCO 111, for preventing the occurrence of the larger phase errors.
However, such high precision clock generating circuit is expensive, thus raising product, ion costs of the reproducing device for the magneto-optical disc to which the clock detection system for the optical disc is applied.